THIRD KEY:Jay Bruce has hit more homers (135) than any player at Great American Ball Park. Maybe the friendly surroundings will continue to warm him up. He is 8-23 (.348) with two homers and four RBI in his last six games.

Rafael Montero was brought up and will start today for the Mets in place of Jacob deGrom. At one time, Montero was in that grouping of the Mets’ young arms, but didn’t pitch well in Triple-A and was demoted.

Montero was 4-6 with a 7.20 ERA at Las Vegas, and was 2-4 with a 10.75 ERA in his last ten starts. He was better at Double-A Binghamton, going 4-3 with a 2.20 ERA in nine starts.

MONTERO: Worth developing. (AP)

Do you remember Jenrry Mejia? The Mets went back and forth using him as a starter or reliever. They bounced him around to the point where he hurt his elbow. In the process his trade value was diminished.

They finally settled on him as a reliever. He panned out in that role, but it was determined his success was chemically related.

Here’s hoping the Mets settle on a role for Montero and stick with it. That’s the best way to showcase him if they are going to deal.

Although the Mets are searching for an explanation for why Montero hasn’t panned out, there’s nothing wrong with his arm, evidenced by a 108 strikeouts to 59 walks ratio.

He’s worth developing, but he walks are an issue as they were in his Aug. 29 start against Miami when he walked a career-high six in five scoreless innings. On a positive note, the walks forced him to pitch out of trouble, which he did. But, the general idea of pitching is to stay out of trouble.

Overall, Montero has made ten career starts in the majors in a spot-start role, going 1-3 with a 3.69 ERA. In that span he has 49 strikeouts to 28 walks.

If Montero can improve his command, he could have a place for the Mets, perhaps coming out of the bullpen. He’s worth the efforts in trying to develop him.

This won’t be a popular suggestion with the Mets’ starters, but with everybody in the rotation having been shelved at one time or another with an injury – save Bartolo Colon – perhaps it might be time to consider a six-man rotation for 2017.

HARVEY: Maybe he could stay healthy in 6-man rotation. (Getty)

None of the young bucks want this, and understandably so because they’ve been raised on the five-man rotation. Change is difficult, but then again one time there was a four-man rotation. This suggestion is prompted by Rafael Montero replacing Jacob deGrom today in Cincinnati, coupled with the report the latter might miss multiple starts.

Matt Harvey is done for the year. So is Zack Wheeler, and nobody knows when he’ll pitch again. After all, it’s been two years now. Steven Matz, sidelined with a bone spur and an impingement in his shoulder, will try to throw in Port St. Lucie today, but his return status is basically a hope.

Noah Syndergaard has also been pitching with a bone spur. Matz’s bone spur will require surgery, but it isn’t known what will happen with Syndergaard.

Meanwhile, deGrom missed time early with a strained lat muscle. His velocity has dipped and after three horrible starts, he has gone from manager Terry Collins not knowing he motioned for the trainer to deGrom saying, “I’m fine,” to missing today’s game, to nobody knows.

The Mets’ rotation for the ages won’t happen this year.

Several weeks ago I wrote how the Mets should re-evaluate the handling of their pitchers. I’m calling for it again, but adding the suggestion they go to a six-man rotation.

Years ago pitchers just pitched. But, the times were different. The salaries have skyrocketed, so there’s a greater need to protect these guys. That’s a partial explanation for why the DL is used so often. What has also changed is pitchers used to throw a fastball, curveball and change-up. Today, there are sliders, sinkers, cut fastballs, all which put strain on the arm.

There’s plenty to share responsibility for Harvey being lost twice, including the player, who wasn’t always upfront. I admire his grit, but we don’t need any heroes. I don’t know if he’ll ever learn, so this might protect him for his own good.

A six-man rotation could save the starters at least a game a month, which is a savings of roughly six starts a year, or as many a 36 innings. Injuries can occur any time despite the greatest precautions, but this could improve the odds or staying healthy.

There will be the natural attrition, such as free agency, trades and injury. Colon might eventually retire. But, if the idea is to keep these guys healthy and pitching, a first step could be reducing the workload.

Some team was the pioneer going from a four-man to a five-man rotation. The Mets have the depth other teams don’t, so why can’t they be the pioneer going to six?

To make this work, it must be installed in spring training with a defined rotation. There can be no deviation, as it will throw off the rest.

If it keeps them off the disabled list, then why not? It’s better than what’s happening now.

Although the Mets trail both St. Louis and San Francisco for a wild-card berth, they still control their own destiny if they run the table.

Neither the Cardinals nor Giants can do the same because they have a four-game series with each other, Sept. 15-18, in San Francisco.

Because or the time difference, it gave me a promotion idea for either the Mets or SNY, both, if they are so inclined.

The 15th of September is Thursday, and with the Mets off that day, why doesn’t SNY arrange to get either the St. Louis or San Francisco feed and show the game that night?

One reader – named AV – just messaged me saying: “I think the Giants feed could be doable if they wanted to do it. The Cardinals games are carried by Fox Sports Missouri but the Giants are on CSN Bay Area, which is a SNY affiliate since Comcast has partial ownership of both channels.”

On the 16th and 17th, after the Mets are done playing the Twins, SNY can again pick up the feeds. At the same time, perhaps the Mets can invite the fans to stay and watch the game on the Citi Field video board, which by the way, has a pretty decent picture.

Keep some of the concessions open and make a party of it. Who knows, perhaps the can arrange for some of the players to stay behind and sign autographs. Cleveland has a postgame autograph signing for its season-ticket holders. I think Tampa Bay has the same.

It sure beats running the bases.

I believe once the Mets invited fans to camp out on the field. A huge slumber party. If the weather is good, why not try it again?

The 18th is a Sunday, and again SNY can pick up the game. If the standings remain as they are, it could create some compelling viewing.

To a player, every year is an audition for the next, and here’s hoping the Mets are taking copious notes on Bartolo Colon. With how well he’s pitched, how ravaged the rotation has been, and the uncertainty of Zack Wheeler’s future, it should be a given re-signing Colon is a priority.

It doesn’t matter he’s 43, or can’t throw his fastball through a wall, or the ceiling of their younger pitchers, Colon knows how to pitch. Colon knows what he has, or more importantly, what he doesn’t possess.

COLON: Bring him back. (AP)

“We had a man on the mound,” manager Terry Collins said. “Nothing fazes him. He gave us what he always does, which is quality innings. He’s an amazing guy.

“Every fifth day he takes the baseball. You don’t have to worry about pitch counts. You don’t have to worry about innings. All he does is make pitches.”

But, none of those pitches were more important than in the third and sixth innings when the Reds had a runner on third with no outs, and twice came away empty. That enabled the half-asleep Mets’ offense time to wake up with three tack-on runs to beat the Reds, 5-0, on Labor Day.

With the victory, the Mets kept heat on St. Louis for the second wild-card and moved to six-games over .500 (72-66), a level they hadn’t been since the night of July 27 when they lost to the Cardinals as Jeurys Familia blew his first save of the season.

The Mets, save Colon, who flew in Sunday afternoon, were dog tired after playing a night game and flying in well past midnight. The Mets were asking Colon to carry them, which he has done now for three seasons.

On Aug. 19, the Mets fell two games below .500 with a loss in San Francisco. Colon beat the Giants the next day to jumpstart the Mets on a stretch where they have won 12 of their next 16 games.

During that stretch, Colon won three games at a time when the Mets lost Steven Matz and Jacob deGrom from the rotation.

Colon gave up five hits and a walk in six scoreless innings to raise his record to a team-high 13-7 with a 3.22 ERA. Colon does it by keeping the Reds off balance by working quickly and staying ahead in the count with a fastball that didn’t stray much over 90 mph.

While Noah Syndergaard throws in the high 90s and sometimes touches triple digits, and deGrom raises red flags when his fastball drops to 91 mph., Colon remains a testament to the pitching tenants of location and movement over velocity.

It’s something the vaunted Mets youthful rotation should learn from, as well as they could from how often to throw between starts. In essence, he’s an active pitching coach.

For as well as Seth Lugo and Robert Gsellman pitched as spot starters, here’s hoping the Mets aren’t seduced by their success and don’t assume Matt Harvey, deGrom and Matz will return without incident. And, for not pitching for the last two years, the Mets can’t assume anything with Wheeler.

However, for the bargain basement cost of $7.25 million, Colon leads the rotation in wins (13), starts (28) and innings pitched (164.2).

Colon doesn’t fit the prototype, but all he does is come through and that’s something that shouldn’t be overlooked.

REYNOLDS RAKES: While everybody was tired, probably nobody was more drained than Matt Reynolds, who flew all night from Salt Lake City and arrived a few hours before game time.

Reynolds caught the red-eye from Salt Lake City to catch a connection in Boston before heading to Cincinnati. And, it didn’t help he was seated next to one of those obnoxious fliers who insist on talking non-stop.

BRUCE RETURNS HOME: Cincinnati will always be home to Jay Bruce, who went 0-for-4 with two strikeouts in his return to Great American Ballpark.

The Reds honored Bruce prior to the game with a video tribute and made a donation to his foundation that supports children with development disabilities.

“It was good. It was a bit odd,” said Bruce. “The Reds took the time to welcome me back. It was what I expected out of this organization. They treated me great the whole time I was here.”

EXTRA INNINGS:Kelly Johnson hit his tenth homer. In looking ahead, the Mets need to seriously consider bringing back Johnson, who doesn’t appear ready to retire. … Wilmer Flores had an interesting day, getting thrown out at second trying to stretch a single and at third attempting to stretch a double. I admit, I was hoping to see him try for an inside-the-park homer. C’mon, admit it, so were you. … The shutout was the Mets’ 11th of the year.